The present invention relates generally to ohmic contacts for semiconductor devices In particular, the present invention is a method for making an ohmic contact for p-type Group II-VI compound semiconductors.
For many years, wide band gap Group II-VI compound semiconductors, particularly ZnSe, have been identified as promising materials for the fabrication of light emitting devices such as laser diodes which operate in the green and blue portions of the optical spectrum. Because of the wide range of important applications for these devices, considerable amounts of research and development have been devoted to these materials. Many major obstacles to the production of commercially viable II-VI devices have been identified as a result of this work. In fact, despite all this research, rudimentary blue light emitting diodes (LEDs) fabricated from an epitaxial II-VI semiconductor (ZnSe) were only first reported in 1988. See e.g., Yasuda et al., Appl. Phys. Lett. 52, 57 (1988). There are no known reports of laser diodes fabricated from these materials.
A significant problem was the inability to p-type dope ZnSe or other appropriate II-VI semiconductor materials to sufficient net acceptor concentrations. Improvements have recently been made in this area. See e.g., the Park et al. U.S. patent application Ser. No. 07/573,428 referred to above and Park et al., P-type ZnSe By Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth, Appl. Phys. Lett. vol. 57, p. 2127 (1990).
Another recent advance in II-VI technology involves growing epitaxial films at low temperatures using molecular beam epitaxy and a thermal-cracking source for the Group VI element. See e.g., Cheng et al., Low Temperature Growth Of ZnSe By Molecular Beam Epitaxy Using Cracked Selenium, Appl. Phys. Lett., vol. 56, p. 848 (1990).
The ability to make low resistance ohmic contacts to both the p- and n-type II-VI semiconductor also presented problems. Good ohmic contacts are necessary for commercially viable (e.g., low operating voltage and low heat generation) II-VI devices.
Conventional techniques for fabricating ohmic metal-semiconductor contacts utilize a metal system (often thermally alloyed) to produce a small barrier to carrier injection, and/or to dope the semiconductor contact layer with shallow (energy level) impurities as heavily as possible at the surface of the layer. Due to the small barrier height and the high doping level in the semiconductor layer, the potential barriers are so thin that tunneling of carriers through the barriers becomes very significant. Most all commercially viable semiconductor devices and integrated circuits employ this approach for current injection.
It was commonly assumed that this technique (e.g., doping and Au evaporation) would also be suitable for producing ohmic contacts to p-type ZnSe and other II-VI semiconductors. In fact, now that low resistance p-type ZnSe can be reproducibly grown, it has been determined that conventional techniques cannot be relied upon to produce acceptable ohmic contacts The stable low-barrier metal system and very high doping levels are, as of yet, not available for these semiconductors. One exception to these problems is ZnTe, which can be easily doped p-type. It is also possible to make ohmic contacts for this semiconductor using conventional techniques. Nonetheless, it is evident that there is a need for improved ohmic contact technology for other p-type II-VI wide band gap semiconductors.